It all depends on what you mean by combine.
If the device doing the combining is linear, (produces no distortion), you will have 100hz and 150hz, the same as you put in.
However, if the device doing the combining is non-linear, you can get all sorts of things:
100, 200, 300, 400...etc
150, 300, 450, 60...etc
This is called harmonic distortion.
100+150=250, 150-100=50 2x250-100=400, 3x250-2x100=550, etc etc.
This is called intermodulation distortion.
It can get quite messy.
JimB